Shear-wave Velocity Structures of the Shallow Crust Beneath the Ordos and Sichuan Basins from Multi-frequency Direct P-wave Amplitudes in Receiver Functions

Abstract:

As the two largest cratonic basins in China, the Ordos basin (OB) and the Sichuan basin (SCB) are of key importance to understand the evolution history of the Chinese continent. In this study, the shear-wave velocity (Vs) structures of the shallow crust (up to 10 km depth) beneath the two basins are imaged based on the frequency-dependence of direct P-wave amplitudes in receiver functions. Teleseismic data used came from 150 broadband seismic stations, most of which were operated for more than one year. The imaging results show different Vs characteristics both between the OB and SCB and within the individual basins. The average Vs for the top 2 km beneath the OB (2.00 km/s) is lower than that of the SCB (2.23 km/s). However, the overall average Vs from surface down to the depth of 10 km is higher in the OB (2.74 km/s) than that in the SCB (2.45 km/s), indicating a larger downward gradient of Vs beneath the OB. Inside the OB, the average Vs above 10 km depth increases from 2.10 km/s in the northwest to 2.65 km/s in the southeast. Compared with the southern part of the OB, the Vs structure in the northern part exhibits a much stronger stratification. The SCB also shows variations in the Vs structure above 10 km depth, with a lower average (2.40 km/s) in the western part than that in the eastern part (2.70 km/s). The thickness of the sediments estimated from the Vs structural image is approximately 5 km and 3 km, respectively, in the west and east of the OB, consistent with geological observations. Compared with the OB, the SCB is covered by a thicker sedimentary layer of more than 7 km in the west and about 6 km in the east. These different structural features of the OB and SCB and the basin interiors may reflect the tectonic imprints of their distinct long-term evolutions since Archean. In particular, previous studies indicated that the two basins experienced different deposition histories and thus had different accumulation-denudation rates of sediments from the Mesozoic to the Cenozoic. This could account to a certain extent for the observed shallow structural differences between the two basins.